The present invention relates to a novel liquid crystalline film and the utilization thereof. Particularly, the invention is concerned with a cholesteric liquid crystalline film and a chiral smectic C liquid crystalline film, with suppressed specular reflection, as well as a novel liquid crystalline film capable of producing a diffracted light having polarizability, and the utilization of those films.
In a cholesteric liquid crystal, internal liquid crystal molecules thereof are oriented in a regularly twisted state so as to describe a helix in a film thickness direction, and the cholesteric liquid crystal possesses a unique optical property derived from the fact that the helical axis is parallel to the film thickness direction. This unique optical property is a selective reflection of a specific circularly polarized light in a specific wavelength band. This property may be applicable to an optical use in which only a right or left circularly polarized light is to be taken out from a non-polarized light or to an ornamental use which utilizes coloration of reflected or transmitted light because the above selection of a circularly polarized light is limited to a specific wavelength band. For industrial applications, a cholesteric liquid crystalline film obtained by forming a cholesteric liquid crystal into a thin film is considered to have a wide application range because it is easy to handle and is superior in processability.
As a method for forming a cholesteric liquid crystalline film there is known, for example, the method disclosed in Japanese Patent Laid Open No. 186534/1994. In conventional cholesteric liquid crystalline films, the helical axis is perpendicular to a substrate, and a layer structure corresponding to a helix pitch forms a flat sheet structure on the substrate. In such conventional cholesteric liquid crystalline films, an incident light is specularly reflected, and the luminance of reflected light drops rapidly outside the specular reflection area because of a marked viewing angle dependence of the luminance.
There also is known an example of having realized a linear polarizing plate by laminating a cholesteric liquid crystal layer and a quarter-wave plate to each other so that the helical axis of the cholesteric liquid crystal layer becomes perpendicular to the quarter-wave plate, as is disclosed in Japanese Patent Laid Open No. 133003/1989. In this polarizing plate, however, an incident light is reflected in an outer polarizing plate due to a specular reflection of the cholesteric liquid crystal layer, and the visibility as a polarizing plate is inferior.
In such a conventional liquid crystalline film as referred to above, the surface thereof has a metallic gloss and is in the state of a mirror surface. In case of applying this film to a use in which reflected light from the film is utilized, the direction of the reflected light is limited to a specific direction due to such specular reflection, giving rise to the problem that a sufficient luminance is not obtainable in other directions. Further, since the selectively reflecting wavelength band of this film is greatly dependent on the viewing angle due to a blue shift phenomenon, the color tone of reflected light varies markedly with the viewing angle.
As a method for suppressing a specular reflection there widely is known a method of diffusing incident light and reflected light. For realizing this method, however, it has so far been necessary to dispose a diffuser panel on the surface of an object to be measured. However, the use of a diffuser panel newly gives rise to problems such as an increase of cost and an increase in the number of manufacturing steps. Therefore, it is desired to develop a cholesteric liquid crystalline film not requiring such a diffuser panel, capable of suppressing a specular reflection, superior in visibility, and less dependent on the viewing angle.
The above points can also be said of a chiral smectic C liquid crystalline film, and also in this case it is desired to develop a chiral smectic C liquid crystalline film not requiring a diffuser panel, capable of suppressing a specular reflection, superior in visibility, and less dependent on the viewing angle.
Next, diffraction gratings are general-purpose optical elements used widely for the purpose of splitting a light beam in the field of spectrooptics. Diffraction gratings are classified into several types according to shapes thereof, usually into oscillation type diffraction gratings in which a light transmitting portion and a light non-transmitting portion are arranged periodically and phase type diffraction gratings in which periodical grooves are formed in a material of a high transmittance. Diffraction gratings are sometimes classified according to directions in which diffracted light is generated (Tetsuo Sueda, xe2x80x9cHow to use Optical Components and Points to be Noted,xe2x80x9d Optronics Co., ISBN4-900474-03-7).
According to the above conventional diffraction gratings, as a diffracted light obtained upon incidence of a natural light (non-polarized light), there can be obtained only a non-polarized light. With use of such a polarizing optical device as an ellipsometer which is used frequently in the field of spectrooptics, there can be obtained only a non-polarized light as a diffracted light, so for splitting a natural light emitted from a light source through a diffraction grating and for utilizing only a specific polarization component contained therein, there usually is adopted a method wherein a diffracted light is used through a polarizer. But this method involves the problem that about 50% or more of the diffracted light obtained is absorbed by the polarizer and that therefore the quantity of light is reduced by half. It is also required to use a detector of a high sensitivity and a light source capable of emitting a large amount of light. Thus, it is desired to develop a diffraction grating through which a diffracted light itself becomes a specific polarized light such as a circularly polarized light or a linearly polarized light.
It is an object of the present invention to solve the above-mentioned problems of the prior art.
The present inventors have succeeded in forming an area which exhibits a high diffusion effect in a cholesteric liquid crystal layer and a chiral smectic liquid crystal layer by controlling the state of orientation of liquid crystal molecules precisely. More particularly, by forming a cholesteric orientation and a chiral smectic C orientation in which the helical axis direction in liquid crystal phase is not uniformly parallel in a film thickness direction, to suppress a specular reflection, we have succeeded in obtaining a cholesteric liquid crystalline film and a chiral smectic C liquid crystlaline film both superior in visibility and having a light diffusing property.
First, by precisely controlling the state of orientation of liquid crystal molecules the present inventors have succeeded in forming an area of a high diffraction efficiency in a cholesteric liquid crystal layer or in a chiral smectic C liquid crystal layer. To be more specific, by controlling and fixing a cholesteric orientation or a chiral smectic C orientation wherein the helical axis direction in cholesteric phase or chiral smectic C phase is not uniformly parallel in a film thickness direction nor is the helix pitch uniformly equal in the film thickness direction, we have succeeded in obtaining a liquid crystalline film which functions suitably as a polarizing diffraction grating.
The present invention is firstly concerned with a light-diffusible cholesteric or chiral smectic C liquid crystalline film having a fixed cholesteric or chiral smectic C orientation in which the helical axis direction is not uniformly parallel in the film thickness direction.
The present invention is secondly concerned with a circular polarizer comprising the above liquid crystalline film.
The present invention is thirdly concerned with a linear polarizer formed by laminating the above liquid crystalline film and a quarter-wave sheet to each other.
The present invention is fourthly concerned with a liquid crystalline film having a fixed cholesteric or chiral smectic C orientation in which the helical axis direction is not uniformly parallel in the film thickness direction nor is the helix pitch uniformly equal in the film thickness direction.
The present invention will be described in detail hereinunder.
In connection with the cholesteric and chiral smectic C liquid crystalline films according to the present invention, reference will first be made below to the cholesteric liquid crystalline film as an example.
In the cholesteric liquid crystalline film according to the present invention, the helical axis direction is not uniformly parallel in the film thickness direction. In one example of such a cholesteric orientation, if the helical axis structure in the ordinary cholesteric orientation is regarded as a pseudo-layer structure, the liquid crystal molecules are cholesterically oriented in an irregularly curved or bent state of the said layer structure. Such a state is generally called a finger-print structure, provided the present invention is not limited thereto.
When a finger-print structure is formed, an oily streak is observed from the surface of the cholesteric liquid crystal layer.
As one mode of the cholesteric liquid crystlalline film according to the present invention there is mentioned a film having such a finger-print structure as referred to above and having a layer with an oily streak formed therein.
A more detailed description will be given below about the said film.
In a cholesteric liquid crystalline film fabricating process according to the present invention, a cholesteric liquid crystalline polymer comprising a liquid crystalline polymer and a predetermined amount of an optically active compound, the liquid crystalline polymer exhibiting a nematic orientability of monodomain and capable of being easily fixed in the state of the said orientation, or a cholesteric liquid crystalline polymer exhibiting a uniform cholesteric orientability of monodomain and capable of being easily fixed in the state of the said orientation, is applied onto an orienting substrate, then dried and heat-treated, allowing a cholesteric orientation to be formed so as to have a finger-print structure and have a layer with an oily streak formed therein, followed by cooling to fix the cholesteric orientation without any damage thereto.
The cholesteric orientation having a finger-print structure and having an oily streak-formed layer, when observed in terms of liquid crystal phase series, is usually present between an ordinary cholesteric phase which forms a flat sheet structure and a liquid crystal transition point as a lower temperature portion thereof. The oily streak-formed layer has a distribution in the thickness direction of the cholesteric liquid crystal film such that the proportion thereof is usually small on the orienting substrate side and large on an air interface side. By utilizing such characteristics, more particularly, by using the air interface side of the film as a light incident surface, the diffusion efficiency of reflected light becomes large and there can be obtained such effects as light-diffusibility, non-specularity and wide visibility.
The following description is now provided about the cholesteric liquid crystalline polymer. As examples of this liquid crystalline polymer, mention may be made of those which exhibit a nematic or cholesteric liquid crystallinity, including main chain type liquid crystalline polymers such as polyesters, polyimides,polyamides, polycarbonates, and polyester-imides, as well as side chain type liquid crystalline polymers such as polyacrylates, polymethacrylates, polymalonates, and polysiloxanes. Above all, polyester type liquid crystalline polymers are preferred in view of easiness of synthesis, orientation and fixing, as well as transparency and glass transition points.
Reference will now be made to the optically active compound which is mixed with a nematic liquid crystalline polymer for imparting twist to the same polymer. Typical examples are optically active low-molecular compounds. Although any compounds are employable in the invention insofar as they are optically active, it is desirable to use an optically active liquid crystalline compound from the standpoint of compatibility with the liquid crystalline polymer. As further examples of optically active compounds are mentioned optically active high-molecular compounds. Although any high-molecular compounds are employable if only they possess an optically active group in the molecule, it is desirable to use a liquid crystalline, optically active high-molecular compound from the standpoint of compability with the nematic liquid crystalline polymer. Examples are such liquid crystalline polymers having an optically active group as polyacrylates, polymethacrylates, polymalonates, polysiloxanes, polyesters, polyamides, polyester amides, polycarbonates, polypeptides, and cellulose. Particularly, mainly aromatic, optically active polyesters are preferred in view of their compatibility with the nematic liquid crystalline polymer.
Thus, as the cholesteric liquid crystalline polymer for forming the cholesteric liquid crystalline film according to the present invention, it is desirable to use a composition comprising a nematic liquid crystlalline polyester and an optically active, low-molecular, liquid crystalline polymer, or a composition comprising a nematic liquid crystalline polyester and an optically active, liquid crystalline polyester. Even other than the compositions comprising nematic liquid crystalline polyesters and optically active compounds, there also may be preferably used cholesteric liquid crystalline polyesters having an optically active group in the main chain.
The cholesteric liquid crystalline film according to the present invention is formed by orienting any of the cholesteric liquid crystlalline polymers exemplified above onto an alignment film formed on a light transmitting substrate and by fixing the resulting orientation, and is usually employed in this state.
As examples of the light transmitting substrate are mentioned glass sheet, light transmitting plastic film, plastic sheet, and polarizing film. As glass there may be used, for example, soda glass, silica-coated soda glass, or borosilicate glass. As plastics of plastic substrates there may be used, for example, polymethyl methacrylates, polystyrenes, polycarbonates, polyether sulfones, polyphenylene sulfides, amorphous polyolefins, triacetyl cellulose, polyethylene terephthalates, and polyethylene naphthalates.
As the alignment film, a rubbed polyimide film is suitable, but there also may be used any of those publicly known in the field concerned. Plastic films or sheets which have been endowed with orientability by being rubbed directly without application of, for example, polyimide thereto are also employable as light transmitting substrates in the present invention. Any orienting process may be used insofar as it allows cholesteric liquid crystalline molecules to be oriented uniformly in parallel with the orientation interface.
Next, a cholesteric liquid crystalline polymer film having a suitable pitch length is formed on the alignment film thus formed on the light transmitting substrate so as to have a finger-print structure and have an oily streak-formed layer.
As means for applying a cholesteric liquid crystalline polymer onto the alignment film there may be adopted a melt application method or a solution application method, the latter being preferred from the standpoint of process.
In the solution application, the cholesteric liquid crystalline polymer is dissolved in a solvent at a predetermined proportion to prepare a solution having a predetermined concentration. The solvent differs depending on the type of the cholesteric liquid crystalline polymer used, but usually there may be used any of such halogenated hydrocarbons as chloroform, dichloroethane, tetrachloroethane, trichloroethylene, tetrachloroethylene, and o-dichlorobenzene, mixtures thereof with phenols, and polar solvents such as ketones, ethers, dimethylformamide, dimethylacetamide, dimethylsufloxide, N-methylpyrrolidone, sulfolane, and cyclohexane. As to the concentration of the solution, it cannot be said sweepingly because it differs depending on what type of a cholesteric liquid crystalline polymer is used, but usually it is in the range of 5 to 50 wt %, preferably 7 to 30 wt %. The solution is then applied onto an alignment film or onto a light transmitting substrate which has been subjected to an orienting treatment such as rubbing.
As the application method there may be adopted, for example, spin coating, roll coating, die coating, or curtain coating.
After the application of the solution, the solvent is removed by drying, followed by heat treatment at a predetermined temperature for a predetermined period of time to complete a cholesteric orientation having an oily streak-formed layer. By cooling the cholesteric orientation to a temperature below the glass transition point of the cholesteric liquid crystalline polymer, the orientation can be fixed without damage thereto.
The cholesteric liquid crystalline film thus formed exhibits a selective reflection phenomenon according to the pitch length upon exposure to light in the infrared, visible, or ultraviolet region. Further, the film is less dependent on the viewing angle and is superior in visibility while a specular reflection is suppressed by the finger-print structure and oily streak formed in the interior of the cholesteric liquid crystal layer.
The application range of the cholesteric liquid crystalline film and the chiral smectic C liquid crystalline film thus prepared according to the present invention and having the above characteristics is extremely wide. For example, they can be used as various optical elements, including polarizing sheets, opto-electronic elements, and materials for ornamental use. As typical and concrete uses are mentioned uses as an optical element which exhibits a selective reflection phenomenon to afford a specific wavelength, an optical filter which cuts off light of a specific wavelength, a circular polarizer, and a linear polarizer combined with a quarter-wave sheet. But these are mere examples.
Particularly, for a use in which specular reflection is not desirable or for a use in which a wide visibility is required, the cholesteric liquid crystalline film or chiral smectic C liquid crystalline film according to the present invention affords an extremely outstanding effect of improvement in comparison with the conventional like films.
Now, description will be directed below to a novel liquid crystalline film capable of generating a diffracted light having polarizability according to the present invention.
In this liquid crystalline film according to the present invention, the helical axis direction is not uniformly parallel to the film thickness direction nor is the helix pitch uniformly equal in the film thickness direction. As an example of such a liquid crystalline film, mention may be made of one having an orientation such that when the helical axis structure in an ordinary cholesteric or chiral smectic C orientation is regarded as a pseudo-layer structure, this layer structure is irregularly curved or bent. The present invention is not limited to such a structure, nor is placed any special limitation on the means for forming such a structure.
As an example of a process for fabricating the liquid crystalline film having such a unique liquid crystal phase structure there is mentioned a process wherein the foregoing liquid crystalline film with fixed cholesteric or chiral smectic C orientaiton having a helical axis uniformly parallel to the film thickness direction and having a uniformly equal helix pitch is formed and thereafter a desired diffraction pattern is transferred to the film. For the pattern transfer there may be adopted, for example, a method wherein a form having a diffraction pattern is provided and is transferred to the film mechanically. In this case, it is not that concaves and convexes of the diffraction pattern are transferred to only the film surface, but it is important for the liquid crystal structure in the interior of the film to be deformed in such a manner that the helical axis is not uniformly parallel in the film thickness direction nor is the helix pitch uniformly equal in the film thickness direction. This desired deformation in the interior of the film may be effected by transferring the diffraction pattern to the film under heating.
The transfer of the diffraction pattern may be carried out by a mechanical method involving using a form having the diffraction pattern, making the diffraction pattern side of the form and the surface of the cholesteric or chiral smectic C liquid crystal layer come closely into contact with each other, and allowing the pattern to be transferred to the film under specific heating and pressurizing conditions.
As the form having the diffraction pattern, no special limitation is placed thereon insofar as the form used is not likely to damage the pattern under heating and pressurizing conditions during the transfer. For example, there may be used a diffraction grating having a grating shape formed on an aluminum or polymer layer which is coated onto a substrate such as a glass or metallic sheet or a polymer film. As the form having the diffraction pattern there also may be used a commercially available one such as, for example, a commercial grade ruled diffraction grating manufactured by Edmund Scientific Co., a transmission type diffraction grating film, or a ruled grating manufactured by JOBIN YVON Co., provided no limitation is made thereto.
The xe2x80x9cmechanicalxe2x80x9d method as referred to herein indicates the use of a molding and pressurizing apparatus such as a press, rolling mill, calender roll, laminator, or stamper.
The film and the form having the diffraction pattern are fed to the above apparatus in a closely contacted state of the diffraction pattern side of the form with the cholesteric or chiral smectic C layer surface and are held under predetermined heating and pressurizing conditions for a predetermined certain period of time. Thereafter, the temperature is reduced for cooling to below the glass transition temperature of the liquid crystalline polymer used and then the form having the diffraction grating is released from the cholesteric or chiral smectic C liquid crystal layer, whereby the liquid crystalline film having such a unique liquid crystal structure as described above according to the present invention can be fabricated.
The above heating condition is usually set to a temperature range above the glass transition point of the liquid crystalline polymer used and below the temperature at which an isotropic phase appears. A concrete heating temperature range cannot be said sweepingly because it differs depending on the type of apparatus and liquid crystal used, the form of film, and the material of a diffraction pattern form used, but is usually in the range of 50xc2x0 to 300xc2x0 C., preferably 60xc2x0 to 250xc2x0 C., more preferably 70xc2x0 to 200xc2x0 C., and most preferably 90xc2x0 to 180xc2x0 C.
As the above pressurizing condition there is adopted a pressure range not impairing the shape of the liquid crystal layer and of the form having the diffraction pattern. A concrete pressure range differs depending on the type of apparatus and liquid crystal used, the form of film, and the material of the form having the diffraction pattern, so cannot be said sweepingly, but is usually in the range of 0.3 to 500 kgf/cm2, preferably 0.5 to 400 kgf/cm2, more preferably 1 to 300 kgf/cm2, and most preferably 2 to 200 kgf/cm2.
Further, the period of time for holding the cholesteric or chiral smectic C liquid crystal layer under the above heating and pressurizing conditions differs depending on the type of apparatus and cholesteric liquid crystal used, the shape of film, and the material of the form having the diffraction pattern, so cannot be said sweepingly, but is usually not shorter than 0.01 seconds, preferably 0.05 second to 30 minutes, more preferably 0.1 second to 15 minutes.
As the liquid crystalline polymer there may be used, as noted previously, a cholesteric or chiral smectic C liquid crystalline polymer comprising a liquid crystalline polymer and a predetermined amount of an optically active compound, the said liquid crystalline polymer exhibiting a uniform nematic or smectic C orientability of monodomain on an orienting substrate and capable of being easily fixed in the so-oriented state, or a cholesteric or chiral smectic C liquid crystalline polymer exhibiting a uniform cholesteric or chiral smectic C orientability of monodomain and capable of being easily fixed in the so-oriented state.
By fixing the cholesteric or chiral smectic C orientation in which the helical axis is uniformly parallel in the film thickness direction and the helix pitch is uniformly equal in the film thickness direction, without impairing the orientation, in the manner described above, and by subsequently transferring the diffraction pattern to the resulting liquid crystalline polymer film by the control method previously described, there can be obtained the liquid crystalline film according to the present invention.
As an example of a method for fabricating the liquid crystalline film according to the present invention there is mentioned a method involving transferring a desired diffraction pattern beforehand onto such an orienting substrate as described above, or using a form itself having a desired diffraction pattern as an orienting substrate, applying a liquid crystalline polymer onto the substrate, performing a heat treatment at a predetermined temperature for a predetermined period of time, and subsequent cooling.
The above method is only an example and the liquid crystalline film according to the present invention is not limited by the manufacturing method.
On the liquid crystalline polymer side of the liquid crystalline film thus formed there may be formed an overcoating layer for protecting the liquid crystal surface. The overcoating layer is not specially limited. For example, there may be used an adhesive which exhibits isotropy optically after curing. In case of using such an adhesive, an overcoating layer can be formed by bonding the liquid crystal side of the liquid crystalline film to a substrate having removability through the adhesive and by removing the removable substrate after curing of the adhesive.
The removable substrate is not specially limited insofar as it possesses removability and a self-supporting property. As the said substrate, a plastic film having releasability is usually preferred. The xe2x80x9cremovabilityxe2x80x9d as referred to herein means that the removable substrate can be removed at the interface with the adhesive in a mutually bonded state of the liquid crystalline film and the removable substrate through the adhesive.
The adhesive is not specially limited if only it can bond the liquid crystalline polymer side and the removable substrate with each other and permits removable of the substrate. As examples of the adhesive, if classified according to curing means, are mentioned photocuring type, electron beam curing type, and heat-curing type adhesives. Particularly preferred are acrylic oligomer-based photocuring type and electron beam curing type adhesives, as well as epoxy resin-based photocuring type and electron beam curing type adhesives. As to in what form the liquid crystalline film and the removable substrate are to be bonded together, no special limitation is imposed thereon. But usually an adhesive layer is disposed between the liquid crystalline film and the substrate. The thickness of the adhesive layer is not specially limited, but is usually in the range of 1 to 30 xcexcm. Various additives, such as antioxidant and ultraviolet absorber, may be incorporated in the adhesive insofar as they do not impair the effect of the present invention.
In the liquid crystalline film both diffraction characteristic and polarization characteristic are developed by the layer structure of the liquid crystal molecules present in the interior of the film, so by disposing an adhesive free of any difference in refractive index the film can be laminated to another optical element without impairing its diffraction characteristic and polarization characteristic.
The liquid crystalline film obtained according to the present invention exhibits a selective reflection phenomenon proportional to the helix pitch length against light in the infrared, visible, or ultraviolet region. At the same time, a diffraction phenomenon is developed by the diffraction pattern formed in the interior of the liquid crystal layer, and the diffracted light possesses a circular polarizability. The liquid crystalline film has such unique features not found in the conventional liquid crystalline polymer film. In this liquid crystalline film, moreover, since both diffraction characteristic and polarization characteristic are developed by the layer structure of the liquid crystal molecules in the interior of the film, such diffraction and polarization characteristics of the film are not impaired even if the film is laminated to another optical element through, for example, an adhesive free of any difference in refractive index.
The application range of the liquid crystalline film according to the present invention, which has such unique optical characteristics as referred to above, is extremely wide. For example, it is employable as any of various optical elements, including a polarizing sheet, opto-electronic elements, and ornamental materials. As typical and concrete examples of uses are mentioned uses as an optical device which requires a spectrally split polarized light, a polarizing optical element or an optical filter, which utilizes a diffraction phenomenon to obtain a specific wavelength, a circular polarizer, and a linear polarizer obtained by combining the film with a quarter-wave sheet. But these are mere examples.
Particularly in the use requiring a spectrally split polarized light the liquid crystalline film according to the present invention exhibits an extremely outstanding effect of improvement in comparison with the conventional combination of a diffraction grating and a polarizer.